CMake Update

[mwalcott3]

If ascent is installed to the default path it should now be usable by other CMake projects using the following:

cmake_minimum_required(VERSION 3.12)
project("example")

find_package(ascent REQUIRED)

add_executable(${PROJECT_NAME} src/main.cpp)
target_link_libraries(${PROJECT_NAME} ascent::ascent)

This should fix issue #3

[stephenberry]

Addresses #3